Method for the stabilization of a fluidized bed in a roasting furnace

ABSTRACT

This invention relates to a method for stabilizing a fluidized bed used in roasting by adjusting the oxygen content of the roasting gas in the bed. The fine-grained material for roasting is fed into the furnace above the fluidized bed and the roasting gas, which causes the fluidizing, is fed into the bottom of the furnace through a grate. In this method, the total amount of oxygen in the roasting gas to be fed and the average total oxygen requirement of the material to be roasted are calculated and the ratio between them regulated so that the oxygen coefficient in the bed is over 1.

This invention relates to a method for stabilizing a fluidized bed usedin roasting by adjusting the oxygen content of the roasting gas in thebed. The fine-grained material for roasting is fed into the furnaceabove the fluidized bed and the roasting gas, which causes the fluidizedbed, is fed into the bottom of the furnace through a grate. In thismethod, the total amount of oxygen in the roasting gas to be fed and theaverage total oxygen requirement of the material to be roasted arecalculated and the ratio between them regulated so that the oxygencoefficient in the bed is over 1.

Roasting can be done in several different furnaces. Nowadays however,the roasting of fine-grained material usually takes place with thefluidized bed method. The material to be roasted is fed into theroasting furnace via the feed units in the wall of the furnace above thefluidized bed. On the bottom of the furnace there is a grate, via whichoxygen-containing gas is fed in order to fluidize the concentrate. Theoxygen-containing gas usually used is air. There are usually in theorder of 100 gas nozzles/m² under the grate. As the concentrate becomesfluidized, the height of the feed bed rises to about half that of thefixed material bed. The pressure drop in the furnace is formed by theresistance of the grate and that of the bed. The resistance of the bedis more or less the mass of the bed when the bed is in a fluidizedstate. The pressure drop is in the range of 240-280 mbar.

The roasting of sulfides is described for example in the book byRosenqvist, T.: Principles of Extractive Metallurgy, pp. 245-255,McGraw-Hill, 1974, USA. According to Rosenqvist, roasting is theoxidizing of metal sulfides, giving rise to metal oxides and sulfurdioxide. For example, zinc sulfide and pyrite oxidize as follows:2ZnS+3O₂→2ZnO+2SO₂  (1)2FeS₂+5½O₂→Fe₂O₃+4SO₂  (2)In addition, other reactions may occur such as the formation of SO₃, thesulfating of metals and the formation of complex oxides such as zincferrite (ZnFe₂O₄). Typical materials for roasting are copper, zinc andlead sulfides. Roasting commonly takes place at temperatures below themelting point of sulfides and oxides, generally below 900-1000° C. Onthe other hand, in order for the reactions to occur at a reasonablerate, the temperature must be at least of the order of 500-600° C. Thebook presents balance drawings, which show the conditions demanded forthe formation of various roasting products. For instance, when air isused as the roasting gas, the partial pressure of SO₂ and O₂ is about0.2 atm. Roasting reactions are strongly exothermic, and therefore thebed needs a cooling arrangement.

The calcine is removed from the furnace partially via an overflowaperture, and is partially transported with the gases to the waste heatboiler and from there on to the cyclone and electrostatic precipitators,from where the calcine is recovered. Usually the overflow aperture islocated on the opposite side of the furnace from the feed units. Theremoved calcine is cooled and ground finely for leaching.

For good roasting it is important to control the bed i.e. the bed has tobe of stable construction and have other good fluidizing properties andthe fluidizing has to be under control. Combustion should be as completeas possible, i.e. the sulfides must be oxidized completely into oxides.The calcine has also to come out of the furnace well, i.e. the particlesize of the calcine must be within certain limits. The particle size ofthe calcine is known to be affected by the chemical composition andmineralogy of the concentrate as well as by the temperature of theroasting gas.

Zinc sulfide concentrates handled in zinc roasters have become moreimpure over the course of time. Concentrates are no longer anywhere nearpure zinc blende, sphalerite, but may contain a considerable amount ofiron. Iron is either dissolved in the sphalerite lattice or in the formof pyrite or pyrrhotite. In addition, concentrates often containsulfidic lead and/or copper. The chemical composition and mineralogy ofthe concentrates vary enormously. In this way the amount of oxygenrequired for oxidation of the concentrates also varies, as does theamount of heat produced on combustion. In the technique currently in usethe roaster concentrate feed is regulated according to the temperatureof the bed using fuzzy logic for example. Thus there is a danger thatthe oxygen pressure in the fluidized bed drops too low i.e. that theamount of oxygen is insufficient to roast the concentrate. As a result,the bed does not agglomerate normally but remains too fine and at thesame time the back pressure of the bed may fall too low, because a finebed stops fluidizing and channeling occurs. The real oxygen demand of afluidized bed is unknown, because generally the concentrate mix is notcalculated continuously in advance on the basis of its precisecomposition, nor are there any devices in the bed for measuring theoxygen content. Therefore the operation of a fluidized bed furnace isdifficult to regulate and keep stable.

The particle size of the zinc sulfide concentrates to be treated alsovaries. As a result, it is difficult to know which part of theconcentrate will burn in the bed when and which part above the bedtransported by the exhaust gas. If a significant amount of thecombustion occurs above the bed, less energy is created in the bed thanusual and, depending on the regulation method, this may increase thefeed.

As stated above, it is known from balance calculations and balancediagrams in the literature that copper and iron together and separatelyform oxysulfides, which are molten at roasting temperatures and evenlower temperatures too. Similarly, zinc and lead as well as iron andlead both form sulfides molten at low temperatures. This kind of sulfideappearance is possible and the likelihood grows if the amount of oxygenin the bed is smaller than that normally required to oxidize theconcentrate.

During fluidized bed roasting agglomeration of the product normallyoccurs, i.e. the calcine is clearly coarser than the concentrate feed.The above-mentioned formation of molten sulfides nevertheless increasesagglomeration to a disturbing degree, in that the agglomerates withtheir sulfide nuclei remain moving around the grate. Agglomerates causebuild-ups on the grate and, over the course of time, block the gasnozzles under the grate. It has been noticed in zinc roasters thatbuild-ups containing impure components are formed in the furnaceparticularly in the part of the grate under the concentrate feed units.

In the article by Nyberg, J. et al: Recent Process Improvements in theKokkola Zinc Roaster, Lead-Zinc Symposium 2000, Pittsburgh, USA, Oct.22-25, 2000, pages 399-415, it is stated that the roaster fluidized bedgenerally moves towards an unstable state when the percentage of thefinest fraction in the bed increases. In this case the temperatures ofthe control thermo-elements diverge, as a result of the fact that thebed is too fine for fluidization and that channeling occurs. Inaddition, the back pressure of the bed drops and the feed drops.

The literature contains research on a zinc sulfide oxidation model,which works at extremely low oxygen contents. According to this model,zinc oxide is formed at low oxygen pressures through gas reactions andnot through a solid-gas reaction as normal. This means that condensedzinc oxide is extremely fine. However, the power of the fans below thegrate is not always sufficient to increase gas feed and likewise theamount of oxygen. On the other hand, the acid plant after the roastermay also cause capacity limitations. The concentrate may also be sofine, that if the gas feed is increased, the material will no longerstay in the fluidized bed but instead will fly out in the flow of gas.Sometimes the quality of the concentrate does not allow changes in thetemperature of the bed and with it the reduction in feed and by thismeans the increase in the amount of oxygen to a sufficient level. Theremay also be situations where neither of the above regulating methods ispossible.

Different ways of regulating roasting conditions have been attempted.U.S. Pat. No. 5,803,949 relates to a method of stabilizing the fluidizedbed in the roasting of metal sulfides, where stabilizing occurs bycontrolling the particle size of the feed. In U.S. Pat. No. 3,957,484stabilization occurs by feeding the concentrate as a slurry. In thearticle MacLagan, C. et al: Oxygen Enrichment of Fluo-Solids Roasting atZincor, Lead-Zinc Symposium 2000, Pittsburgh, USA, Oct. 22-25, 2000,pages 417-426, it is stated that the oxygen content of the roasterexhaust gas is controlled by measurements taken from the gas line afterthe boiler or the cyclone. These measurements do not, however, tell ofthe status of the fluidized bed, because the gas line measurementsalready include leakage air.

In order to correct the deficiencies presented above, a method accordingto the present invention has now been developed to stabilize a fluidizedbed for use in roasting fine material by regulating the oxygen contentof the gas in the bed. In order that for instance zinc sulfideconcentrate be oxidized into zinc oxide, the oxygen coefficient of thefluidized bed should in theory be at least one. The oxygen coefficientis obtained when the total oxygen feed of the roasting gas is calculatedand compared to the total oxygen requirement of the concentrate feedmixture. According to the method now developed, the oxygen coefficientis adjusted to be over 1, preferably at least 1.03. In order to effect amore accurate adjustment, the oxygen content is also measured in the beditself. The stabilization of the fluidized bed by regulating the oxygencoefficient prevents capacity losses, which result from the build-upformed on the grate and the production stoppages they cause. Theessential features of the invention will be made apparent in theattached claims.

According to the present method, it is possible to do the adjustment ofthe oxygen coefficient on the basis of two process data: first calculatethe average oxygen requirement of the feed mixture (NM³O₂/t concentratemixture) using the calculated oxygen requirements of the studiedchemical and mineralogical composition of the each concentrate. Theoxygen requirement of the concentrate mixture is entered into theprocess control equipment whenever the mixture is changed. The secondprocess data required is the total oxygen requirement, which iscalculated on the basis of the oxygen requirement of the feed mixtureand the concentrate feed (t/h) to be measured continuously. Duringroasting, the process control equipment measures the oxygen coefficientof the process i.e. it compares the total oxygen feed to the calculatedtotal oxygen requirement. The total oxygen feed is obtained by measuringthe amount of gas to be fed via the grate and its oxygen content. Thecontrol equipment is given appropriate limit value, and if the oxygencoefficient falls below this limit, the equipment reacts in theprescribed manner e.g. with an alarm or a certain adjustment procedure.These kinds of adjustment procedures are, depending on the situation,the adjustment of the oxygen coefficient to the right range, either bychanging the temperature, the amount of grate air or oxygen enrichmenteither separately or together in different combinations. Pure oxygen maybe fed with the grate gas as oxygen enrichment.

As stated previously, with embodiments of the prior art of roasting ithas not been able to determine which part of the concentrate will beoxidized in the bed and which part only above the bed and what thepercentage of leakage air will be. Thus there is no precise picture ofthe sufficiency of the amount of oxygen in the bed. Therefore, in orderto specify the adjustment action, it is necessary to carry out oxygencontent measurement in the bed also. In the present invention thefine-adjustment of oxygen content can be done either continuously or forexample only when changing the feed mixture. Probes for instance areused as the measurement device. On the basis of this measurement, theactions described above are carried out as required in order to adjustthe oxygen coefficient to the right range. In particular when usingoxygen enrichment the avoidance of wasted costs should be kept in mindor feeding oxygen in excess, since pure oxygen is expensive.

The invention is described further in the following example:

EXAMPLE 1

A concentrate with a sphalerite composition was compared to a zincconcentrate containing pyrite. Calculating the oxygen requirement of theconcentrates showed that the oxygen requirement of the sphaleriteconcentrate in roasting is 338 Nm³/t and for the pyrite-containingconcentrate 378 Nm³/t, in other words the oxygen requirement of thepyrite-containing concentrate is over 10% greater than that of thesphalerite concentrate. The mineral contents of the concentrates areshown in Table 1.

TABLE 1 Pyrite-containing Sphalerite concentrate concentrate Mineral w-%w-% CuFeS₂ 0.09 1.73 FeS 2.54 2.85 FeS₂ 0.35 21.63 ZnS 91.66 68.11 PbS 13.11 CdS 0.24 0.18 SiO₂ 0.94 0.43 CaSO₄ 0.83 0.1 CaCO₃ 1.05 0.5 others1.3 1.36

1. A method of stabilizing a fluidized bed used in roasting of afine-grained material, comprising: calculating the total amount ofoxygen in the roasting gas to be fed; calculating the average totaloxygen requirement of the material to be roasted; measuring the oxygencontent within the fluidized bed; and regulating a ratio between thetotal amount of oxygen in the roasting gas to be fed and the averagetotal oxygen requirement of the material to be roasted so that theoxygen coefficient in the bed is greater than
 1. 2. A method accordingto claim 1, wherein the oxygen coefficient is adjusted to be at least1.03.
 3. A method according to claim 1, wherein the oxygen coefficientis adjusted by changing the temperature by adjusting an amount ofmaterial to be roasted.
 4. A method according to claim 1, wherein theoxygen coefficient is adjusted by changing the amount of roasting air.5. A method according to claim 1, wherein the roasting gas is air.
 6. Amethod according to claim 1, wherein the oxygen-enriched air is used asthe roasting gas.
 7. A method according to claim 6, wherein the oxygencoefficient is adjusted by changing the oxygen enrichment of theroasting gas.
 8. A method according to claim 1, wherein the oxygencontent measurement from the bed is made continuously.
 9. A methodaccording to claim 1, wherein the oxygen content measurement from thebed is carried out when changing the feed mixture.
 10. A methodaccording to claim 1, wherein the material to be roasted is a zincconcentrate.
 11. A method according to claim 1, wherein the material tobe roasted is an iron-containing sulfide concentrate.